Electron-Withdrawing Substituents Decrease the Electrophilicity of the Carbonyl Carbon. An Investigation with the Aid of¹³C NMR Chemical Shifts, ν(CO) Frequency Values, Charge Densities, and Isodesmic Reactions To Interprete Substituent Effects on Reactivity

Abstract

(13)C NMR chemical shifts and nu(C[double bond]O) frequencies have been measured for several series of phenyl- or acyl-substituted phenyl acetates and for acyl-substituted methyl acetates to investigate the substituent-induced changes in the electrophilic character of the carbonyl carbon. Charge density, bond order, and energy calculations have also been performed. The spectroscopic and charge density results indicate that opposite to the conventional thinking, electron-withdrawing substituents do not increase the electrophilicity of the carbonyl carbon but instead decrease it. On the other hand, reaction energies of the isodesmic reactions designed show that electron-withdrawing substituents destabilize the carbonyl derivatives investigated. So, a significant ground-state destabilization of carboxylic acid esters, and carbonyl compounds in general, due to the decreased resonance stabilization, is proposed as a novel concept to explain both the increase in their reactivity and the changes in the chemical shifts and carbonyl frequencies induced by electron-withdrawing substituents.

1,3-Dichloro-5,5-dimethylhydantoin as a Novel and Efficient Homogeneous Catalyst in Biginelli Reaction

“…But, the yields decrease slightly with electrondonating substituted benzaldehydes (entries f-i, l) due to their stability rather than electron-withdrawing substituted ones. 16 The applicability of the current method for the large scale synthesis was also examined by the reaction of 5 mol benzaldehyde with 5 mol ethyl acetoacetate and 7.5 mol urea in the presence of 0.25 mol DCDMH. The product was isolated in 91% yield after 4 hours.…”

The case of intramolecular hydrogen bonding, hyperconjugation and classical effects on the conformational isomerism of substituted carbonyl and thiocarbonyl compounds

“…ICS (Intramolecular Interaction Chemical Shifts) and SCS (Substituent induced Chemical Shifts) values did not correlate well with usual substituent electronic and/or steric parameters in mono-substituted acetic acids, and this has been supposed to be due to orbital interaction involving the carboxyl group [6]. NMR coupling constants and infrared intensities have also demonstrated to be dependent on conformation of ketones and esters, and their conformational balance is assumed to be much due to orbital interactions [7][8][9][10][11][12][13].…”

Influence of Aprotic Solvents on the Transmission of Anomalous Substituent Effects on 13C NMR Chemical Shifts at the Carboxyl Carbon (δco) in Meta-Substituted Benzoic Acids: A Strong Evidence for π-Polarization Mechanism

“…Considerable progress has been made towards understanding the physical basis of these shifts [8][9][10][11][12][13][14][15][16], their dependences on substituent effects [8,10,12,[16][17][18][19][20][21][22][23][24][25][26][27][28], and their correlations with the substrate reactivity [22,[25][26][27][28]. An interesting finding is the anomalous (reverse) substituent effect on the chemical shift at the a-carbon ðd C a Þ of an unsaturated side chain (containing a C=C, C:C, C=N, C:N, or C=O group) of an aromatic compound: an electron-withdrawing substituent causes an upfield shift at the a-carbon [8,10,12,[16][17][18][19][20][21][22][23][24][25] in contradiction to the expected decrease in the electron population there.…”

“…An interesting finding is the anomalous (reverse) substituent effect on the chemical shift at the a-carbon ðd C a Þ of an unsaturated side chain (containing a C=C, C:C, C=N, C:N, or C=O group) of an aromatic compound: an electron-withdrawing substituent causes an upfield shift at the a-carbon [8,10,12,[16][17][18][19][20][21][22][23][24][25] in contradiction to the expected decrease in the electron population there.…”